A common way of manufacturing a cylindrical format li-ion cell is to wind a stack of electrodes interleaved with separator material into a spiral structure commonly termed a jelly roll to form an electrochemical cell. The jelly roll is comprised of, for example, anode and cathode electrodes, and a layer of separator material between each anode and cathode. Strips of conductive material, generally referred to as current collecting “tabs” are welded to the anodes and cathodes to provide a means by which current can pass to and from the electrodes. Once the anode, cathode and separator materials are wound, the tabs will protrude from both ends of the jellyroll.
To facilitate manufacturing of electrochemical cells, the current collecting tabs are often made longer than what is required by the final geometry of the cells, which allows the tabs to be folded to provide sufficient length to prevent fatigue of the material. The excess length can result in a short circuit and a non-functioning product. A short circuit can also be caused by a tab's movement due to cell cycling, mechanical shock, and/or vibration loading during the lifetime of the cell. To prevent short circuits, a circular tab insulator with one or more holes that allow tabs to pass through can be used. However, because the relative positions of the tabs can vary, it is difficult to accommodate multiple tabs with the insulator, while providing for an efficient and accurate assembly into an electrochemical cell. Even when the tabs are aligned with each other, installation of an insulator can be difficult. Once the insulator is installed the difficulty can continue with bending the tabs in the correct location. For example, if the tab bend occurs below the level of the separator it can introduce additional stress that may cause eventual failure.
The tabs can be covered with adhesive backed polyimide material, while using additional strips of polyimide tape over the battery cell to prevent the tabs from contacting the cell. However, polyimide tape adhesives soften considerably with exposure to heat or lithium ion cell electrolyte. This allows the tape to move away from its protective position during mechanical shock and vibration loads typical of hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) applications. Even a perfectly formed tab is subject to movement due to cell cycling, mechanical shock and vibration loading during the lifetime of the cell. Because of the short circuit risk, an insulative member may be employed on either end of the jelly roll.
A known insulator 10 is shown in FIG. 1 and used to accommodate four tabs 12, 14, 16, 18 of an electrochemical cell. Step 1 of FIG. 1 shows the rolled top face of a battery and the location of all four tabs as they project from the face of the jellyroll. The insulator 10 is positioned over the end of the jellyroll, and the tabs 12, 14, 16 and 18 are slid through slots 20 in the insulator 10. Afterwards, all four tabs 12, 14, 16 and 18 are bent towards the center axis of the jellyroll over the insulator 10, as shown in step 2. The result is a stack of tabs fanning in a region of up to about 140 degrees over of the face of the jellyroll. The tab length may vary, such that the tab closest to the center axis, e.g., tab 18, is the shortest and the tab farthest from the center axis, e.g., tab 12, is the longest.
The tab closest to the center of the jellyroll may be cut to a shorter length than the rest of the tabs, and each subsequent outwardly positioned tab is longer than the previous inner tab. The result is that when all four tabs are folded over, as illustrated in step 2, their ends align the same distance away from the axis of the jellyroll. Once the tabs are lying flat, they are ready to be connected to a terminal of the electrochemical cell. This is accomplished by bending the tabs at an angle, as illustrated in step 3, to consolidate the four tabs into one entity to which the cell's extension tab can be welded. This type of insulator is commonly installed by hand during the manufacturing process. In the completed electrochemical cell, the insulator 10 helps prevent the current collecting tabs 12, 14, 16 and 18 from contacting the electrode of the opposite polarity thereby avoiding a short circuit. Tabs close to the center may initially be bent away from the core, then back towards the core to accommodate longer tabs.